THE ENERGY OF THE PLANT 281 
Finally we have evidence of the power of plants to 
avail themselves of the heat rays. The relations existing 
between the organism and its environment have already 
been mentioned. Not only can the air rob the plant of 
heat by radiation, but when its own temperature is high it 
- can communicate heat to it in turn. Leaves have been 
proved to absorb heat with great avidity, particularly those 
which are succulent or fleshy, a difference of more than 
20° C. having been noted between their temperature and 
that of the air. The direct absorption of the rays of heat 
from the sun has also been noted, apart from the tempera- 
ture of the air through which the rays were passing. 
The supply of radiant energy is very much in excess of 
the amount which is needed for the internal work. Indeed 
its absorption by the leaves would be a source of consider- 
able danger to the plant were it not for the cooling effect 
of transpiration, which we have seen dissipates 98 per cent. 
of it during bright sunshine. No doubt this dissipation is 
one of the chief benefits secured by transpiration. 
It is evident, however, that in the general economy 
of the plant something further must be at work in connec- 
tion with the supply of energy. The absorption of these 
external forms must take place at the exterior of the plant, 
while many of the processes of expenditure are carried out 
in parts which are more or less deep-seated. Weare obliged 
to turn our attention, therefore, in this connection as in 
that of the construction and utilisation of food, to processes 
of accumulation, distribution, and economy. 
We may ask ourselves what is the immediate fate of 
the energy absorbed. It enters the plant in what is known 
as the kinetic form. A very considerable part of the 
kinetic energy of the sun’s rays, we have already seen, is 
devoted at once to the evaporation of the water of transpira- 
tion, but some of it is employed by the chloroplasts to con- 
struct some form of carbohydrate. The energy so applied 
can be again set free by the decomposition of this formed 
material. If the latter were burned its combustion would 
